A typical packaged integrated circuit unit includes a die on which the integrated circuit is formed and a package substrate on which the die is mounted on. An interconnect structure connects the terminals of the die from the integrated circuit in the die to the terminals of the package, which can be further connected to other components through a circuit board. The package may be directly mounted on the circuit board, or through a socket or an interposer.
A sophisticated Integrated Circuit (IC) chip such as a microprocessor uses a large number of terminals for connection to a circuit board. Various packaging techniques have been developed to provide the terminals for connection to the circuit board with an increasing number of terminals on a small size package. A grid array type of packaging technique arranges the terminals in a grid array pattern on one side of the package, such as a Pin Grid Array (PGA) package, a Ball Grid Array (BGA) package, a Column Grid Array (CGA) package, or a Land Grid Array (LGA) package. These terminals are typically based on flat pads formed on a dielectric layer of the package. For example, in a land grid array package a solder resist opening defines the land out of a portion of a pad; in a ball/column grid array package a solder ball/column is attached to the pad through a solder resist opening. Other types of terminals, such as pins, can also be used.
When a flip chip packaging technique is used, a die is flipped so that the terminal side of the die faces the substrate and the terminal side of the package. An interconnect structure is typically formed on the substrate to connect the terminals of the die to the terminals of the package. Underfill materials are typically used between the die and the substrate to provide bonding strength and physical protection for the connection between the die and the substrate against external force, moisture, etc.
Mismatch in coefficients of thermal expansion of the die, the underfill and the package substrate can lead to warpage of the package substrate. Typically, the underfill materials are applied at an elevated temperature to fill in the space between the die and the substrate, surrounding the electric connections (e.g., solder connections) between the die and the substrate. When the package is cooled down to a room temperature, differences in thermal expansion in the die, the underfill and the substrate can cause the package to bend toward one side.